The present invention relates to methods and arrangements in a telecommunication system, and more particularly to methods and arrangements for efficiently utilizing radio resources to communicate channel quality information from a user equipment to a network node in the telecommunication system.
Wireless mobile communication systems often rely on some form of arrangement such as that illustrated in FIG. 1. As shown, a base station 101 communicates with a mobile device (which is often generically referred to in the art as a user equipment (UE)) 103, via a multipath channel 105. Because the signal transmitted (e.g., by radio waves) by the base station 101 spreads out into the environment and these spread signal components are often reflected off of any number of objects, the transmitted signal reaches the UE 103 via more than one path. Therefore, in addition to the direct path 107 from the base station 101 to the UE 103, there are a number of other indirect paths 109. The contributions from these indirect paths exhibit different signal attenuations and time delays relative to that from the direct path, and these contributions may interfere with the contribution from the direct path either constructively or destructively at the UE's receiver input. The signal components caused by the direct path 107 and the indirect paths 109 are denoted multipath components and can be modeled by a complex channel filter function either in the time domain as a channel impulse response or, correspondingly, as a channel frequency response in the frequency domain.
FIG. 2 is a block diagram illustrating in greater detail a number of aspects of some mobile communication systems. In this example, so-called multiple-input, multiple-output (MIMO) communication technology is used. MIMO systems employ multiple antennas at the transmitter and receiver to transmit and receive information. The receiver can exploit the spatial dimensions of the signal at the receiver to achieve higher spectral efficiency and higher data rates without increasing bandwidth. As shown in FIG. 2, a base station 201 communicates with a UE 203 via a multipath channel 205. In downlink transmissions, an information signal, I(t), (e.g., in the form of a binary data stream) is supplied to the base station 201. The base station 201 includes a controller 207 and a transmit signal processing circuit 209. The controller 207 controls operation of the base station 201 and schedules UEs 203 to receive data on a downlink channel. The transmit signal processing circuit 209 performs such functions as error coding, mapping the input bits to complex modulation symbols, and generating transmit signals for each of one or more transmit antennas 211, which may be independent, partially redundant, or fully redundant. After upward frequency conversion, filtering, and amplification, the base station 201 transmits the transmit signals from respective transmit antennas 211 through the channel 205 to the UE 203.
Receiver equipment in the UE 203 demodulates and decodes the signal received at each of its antennas 213. The UE 203 includes a controller 215 to control operation of the UE 203 and a receive signal processing circuit 217. The receive signal processing circuit 217 demodulates and decodes the signal transmitted from the base station 201. In the absence of bit errors, the output signal from the UE 203, Î(t), will be the same as the original information signal I(t).
While FIGS. 1 and 2 generically illustrate components of a mobile communication system, implementation details can vary from one system to another. A number of different standards that govern the design and operation of mobile telecommunication systems are known. For example, High Speed Downlink Packet-data Access (HSDPA) is an evolution of Wideband Code Division Multiple Access (WCDMA) specified in the Release 5 version of the Third-generation Partnership Project (3GPP) WCDMA specification. HSDPA introduces higher bit rates (up to over 10 Mbits/s) by using higher order modulation (16-QAM), multicodes (up to 15 with spreading factor 16), and downlink channel feedback information. Downlink channel feedback information is information, sent to the base station, regarding the downlink channel quality. The base station (BS), which in 3GPP terminology is called “node B”, uses this information to optimize modulation and coding for optimized throughput. Furthermore, Hybrid ARQ is also introduced on the physical layer in order to reduce the round trip delay for erroneous received packets.
HSDPA works according to the following. A User Equipment (UE), operating in connected mode, continuously transmits Channel Quality Index (CQI) reports to the HSDPA serving node B by means of the uplink (UL) High Speed Dedicated Physical Control Channel (HS-DPCCH). The CQI informs the serving node B about the instantaneous downlink (DL) channel quality in order to enable the node B to optimize the downlink throughput. The CQI could, for example, be a function of Signal to Interference Ratio (SIR), where the particular function depends on higher layer parameters (e.g., available HS-power, and the like). When the UE is scheduled by the node B and data packets will be transmitted to the UE, the HS Shared Control Channel (HS-SCCH) is used to inform the UE about information that the UE will use in the upcoming communication, such as information about the data packets and transport format, retransmission number, and the like.
The downlink packet-data enhancements of HSDPA are complemented by “Enhanced Uplink”, introduced in Release 6 of the 3GPP/WCDMA specifications. HSDPA and Enhanced Uplink are often jointly referred to as High-Speed Packet Access (HSPA).
To take another example, a new flexible cellular system, called Third Generation Long Term Evolution (LTE), can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.4 MHz to 20 MHz. Furthermore, data rates up to and exceeding 100 Mb/s will be supported for the largest bandwidth. However, it is expected that LTE will be used not only for high rate services, but also for low rate services like voice. Since LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VOIP) will likely be the service that carries speech.
Like HSPA, in LTE systems channel quality information is computed at the UE. This computation is based on reference pilot data transmissions, and is used for link adaptation (e.g., determining modulation and coding rates for the scheduled user). In these and other similar systems, the base station typically retains control for making decisions regarding user scheduling and link adaptation because the base station has the knowledge of the CQI reports from all users as well as specific data transmission requirements for each user.
The inventors of the inventive subject matter described herein have noted that there is a delay between when the mobile computes the CQI information and when the base station makes a downlink transmission based on the corresponding CQI report. This delay results in a potential mismatch in the CQI at the time of transmission because the channel may have changed in the time between CQI reporting and the subsequent downlink transmission. This mismatch will be greater for higher fading rates, but there may still be some mismatch for lower rates. One method for compensating for this mismatch is to predict what the fading channel will be at a time when the downlink transmission will occur, and compute the CQI based on this predicted channel. Such an approach is described in PCT Pub. No. WO2007/032715, which also discloses techniques for a first network unit of a communication system estimating the reliability of the predicted channel estimate that is to be used for link adaptation, and communicating this reliability estimate to a second network unit. The second network unit can then take this information into account when deciding on, for example, selection of a transmission scheme for use when communicating with the first network unit.
Another reason why a reported CQI value might not be a reliable basis upon which to make scheduling and link adaptation decisions is interference that varies from one time to another.
Despite their imperfect nature, CQI reports are nonetheless an important factor in the quality of mobile telecommunications service. There are a number of issues with CQI reporting that are considered below:                The CQI reports are considered a fixed resource, with mobiles making CQI reports periodically at some scheduled rate. As the number of mobiles increases within a system, the number of CQI transmission increases, placing a greater load on the uplink resources.        More advanced multi-antenna transmission schemes (e.g., MIMO) are being introduced that require a greater amount of feedback per mobile, thus increasing the load on the uplink feedback rate even further. This effect can be partially reduced by an approach described in US Patent Pub. 2005/0143084, which selects between either the single-antenna CQI report or the multi-antenna CQI report based on some criteria to control the system's CQI feedback rate. Thus, while there are different CQI reports corresponding to different mobile classes, a baseline CQI report is still made from each mobile.        An estimate of the channel prediction error reliability can also be included as part of the CQI report as described in the above-referenced PCT Pub. No. WO2007/032715. However, this further adds to the load on the uplink feedback rate. Alternatively, the channel prediction error reliability can be computed at the base station, but this requires channel estimates to be available at the base station.        
In CDMA systems, the increased load on the uplink feedback rate has the effect of increasing the so-called noise-rise, which is the apparent interference-plus-noise level observed by the base station.
Another issue is related to the time-varying nature of the CQI estimate. The CQI report contains a fixed number of bits that is transmitted from the mobile to the base station during every CQI reporting period. This occurs regardless of whether the CQI values have changed significantly (or not) from one CQI report to the next.
It is therefore desirable to provide methods and apparatuses that efficiently utilize radio resources to communicate channel quality information from a user equipment to a network node in the telecommunication system